1. Field of the Invention
This invention relates to injection molding machines, and more particularly to the injector portion which conveys molten liquid plastic material from a source of liquid plastic to the mold cavities of the injection molding apparatus.
2. Prior Art
Injection molding machines are widely used for the rapid and repeated molding of articles from thermoplastic materials, such as polyethylene, polypropylene, polyethylene terephthalate, and polyvinylchloride. Such machines typically include an injector portion in which a molten thermoplastic material is injected into a mold cavity to form an article, and an ejector portion which withdraws the cooled molded article from the cavity and ejects it into a catcher or similar receptacle.
The injector portion of the machine typically includes a hopper for receiving beads of a selected thermoplastic material which communicates with a reciprocating helical screw mounted within a heated casing. The screw conveys the beads of thermoplastic material along the heated casing at which time the beads melt and become a liquid mass. The casing necks down to an orifice which communicates with a manifold, and reciprocation of the screw projects a charge of the material into the manifold. The manifold receives the heated and liquified plastic material from the orifice into a sprue and conveys it through runners to a plurality of gates. Each gate communicates with a mold cavity and the plastic material flows into the cavity to form the molded article.
The ejector portion of the injection molding machine typically includes a plurality of cores shaped to fit into the cavity and about which the molded article is formed. The cores are mounted on a frame which is mounted on a reciprocating platen so that the cores are movable into and out of the cavities. The frame may include means, such as movable jaws, for gripping the cooled molded article so that the article will remain on the core and will be removed from the cavity when the cores are withdrawn from the cavity. The gripping means may also be capable of ejecting the solidified article into a catcher or other receptacle which removes the articles from the injection molding machine to a remote location for packing and shipping.
Much consideration has been given to the arrangement and construction of the aforementioned manifold, and to manifolds which can convey the molten plastic from a single outlet orifice to a plurality of cavities without the plastic freezing or solidifying in the manifold runners.
For example, U.S. Pat. No. 2,672,653 discloses a mold manifold which forms a plurality of runners connecting the inlet sprue to a plurality of outlet gates. The manifold also includes bores for receiving heating elements to insure that the liquid plastic does not solidify within the runners. However, manifolds of this design possess a disadvantage in that the lengths of the runners between the inlet sprue and the outlet gates are not equal. Therefore, the time required for plastic material entering the inlet sprue to flow to the mold cavities varies from cavity to cavity.
In addition, the location of the heating elements is undesirable in that the distances from the heating element to the runners are not constant. Therefore, portions of the runners will receive more heat energy than others, resulting in uneven heating. Such an arrangement of a heating element is undesirable also in that greater total heat energy is required to raise the temperature of a manifold to achieve the minimum required temperature in those portions of runners or outlet gates which are more distant from the heating element, than if all portions of runners and all gates were equidistant from a heat source. This results in wasted heat energy and higher operating expenses.
Another type of manifold is disclosed in U.S. Pat. No. 3,520,026. This patent is directed to a manifold for an injection molding machine which is itself a resistance heating element. The manifold contains two runners which are formed such that the distance from the inlet sprue to the outlet gate is the same for each runner. However, such a manifold design possesses an inherent disadvantage of wasting much heat energy which is radiated to the surrounding environment. Since the air surrounding the heated manifold is generally cooler than the liquid plastic material flowing within the heated manifold, the rate of heat flow outward to the surface is greater than the rate of heat flow inward to the runners. In addition, the patent discloses a manifold for conveying plastic from a single inlet sprue to outlet gates but does not disclose a means for conveying plastic to a plurality of outlet gates in such a manner that the runners are of equal length.
Another example of a manifold having runners of equal length is disclosed in U.S. Pat. No. 4,219,323, assigned to the assignee of the present invention. This patent discloses a manifold in which the runners are heated by a plurality of heating elements positioned within bores in the manifold. The necessity of a plurality of heating elements is undesirable in that it adds to the expense and complexity of the manifold.
In the manufacture of parisons which are later blow molded to form beverage containers, an additional problem is encountered. If the molten plastic material, typically polyethylene terephthalate, is allowed to remain in a heated liquid state for too long a time, the amount of acetaldehydes formed prior to cooling will be sufficient to taint the flavor of the beverage to be carried in the container. Therefore, the length of the runners in a manifold must be minimized to shorten the heat history of the plastic material.
In injection molding of parisons, it is desirable to have a large number of mold cavities in operation with a minimal amount of downtime for repair or replacement of the mold cavities. Typically, a single injection molding machine may have from 20 to 50 cavities in continuous operation during a molding operation. The breakdown of a single cavity may in some cases necessitate the interruption of the operation of the entire machine in order to effect repair.
Accordingly, there is a need for a manifold for use with an injection molding machine that is capable of conveying heated liquified plastic from an inlet sprue to a plurality of cavities in such a manner that the plastic is maintained in a liquid state with a minimum of energy expenditure and complication. There is a need for a manifold with runners of a minimal length to minimize the heat history of the material conveyed, and, there is a need for a manifold in which the runner paths from the inlet sprue to each outlet gate are of the same length. There is also a need for an injection molding machine in which the breakdown of a single mold cavity does not cause extensive downtime of the apparatus to effect repair.